Formation evaluation in high-angle and horizontal (HA, HZ) wells is still a challenge due to measurement complexity, environmental effects and lack of adequate interpretation answer products. The effects of bed boundary crossings at high angles, influence of adjacent bed above and below the wellbore, as well as anisotropy were recognized on all the measurements. In case of resistivity interpretation, these effects made it impossible for the petrophysicists to use any of resistivity measurements as a direct representation of Rt over the intervals where the effects are present. It is necessary to model the tool response in order to determine the individual layer Rt, or to compare directly the offset well values for correlation and infer an Rt for petrophysics. Development of directional measurements significantly increased the potential to accurately characterize formation structure near the wellbore, correct the effects of adjacent boundaries and made possible use of inversion to build more accurate 2D or 3D formation models.
Nuclear density measurements progressed from designs focused on measuring an average density for vertical wells to a fully azimuthal measurement for inclined wells. While being a shallower measurement than the resistivity, the nuclear density is also affected by bed crossings and adjacent beds, and additionally by standoff and asymmetric (“teardrop”) invasion.
Traditionally, modeling of the nuclear tool responses relies on Monte Carlo Nuclear Particle (MCNP) modeling codes that are computationally intensive and not suitable for log analysis. They are primarily used in the tool design process to optimize the sensitivity and accuracy of the measurement and to develop cross-plots, correction charts and case studies to better understand responses in complex scenarios.
The first quantitative inversion-based interpretation of nuclear density images was developed using a fast-forward model based on linear approximation and 2D flux sensitivity functions. The methodology ignores the borehole and determines the initial formation model using only the bottom quadrant data where the standoff is generally negligible. Based on the dip extracted from image sinusoids, the technique is able to build a 1D layered earth model of formation density in high-angle wells. However, its applicability is limited due to the limitations of 2D (axisymmetric) assumption, and the loss of information from not using data from all sectors. The same constraint severely limits the applicability to horizontal wells, where the image usually does not contain sinusoids, and use of bottom sector data is not sufficient in automated interpretation workflows due to variation of dip and effects of adjacent (non-crossed) layers.